ZR-IN-RUTILE THERMOMETRY OF PELITIC SCHISTS IN THE SOUTHERN APPALACHIAN BLUE RIDGE

P-T-t data for southern Appalachian rocks remain scarce; consequently, little is known about the tectonothermal history of the various terranes, including depth of burial, variations along strike, and rates of heating, burial, cooling, and exhumation. Here we report initial results from our sampling of rutile-bearing pelitic schists in the Blue Ridge. EMP analyses of rutile for trace elements (Si, Cr, Zr, Nb, Ta) allow calculation of Zr-in-rutile temperature estimates. In samples of Western Blue Ridge gar-ky-st schists near Bryson City, NC, Zr-in-rutile Ts yield consistent estimates of 589±18 and 600±26 °C, similar to previously published results for this area. Garnets in these rocks are almandine-rich (Alm₆₀Sps₂₀Pyp₁₅Grs₅) and show fairly flat patterns in major element zoning, with minor retrogression in Fe-Mg contents. At higher grades in sillimanite-bearing rocks, however, T estimates by Zr-in-rutile thermometry show little internal consistency between samples and are much lower than previously published values. Peak conditions of Taconic metamorphism (~460-450 Ma) have been estimated at 850 °C (gar-bio) and 7-9 kbar (GASP) at Winding Stair Gap, however, our samples from Winding Stair Gap yield scattered, variable, and significantly lower estimates of 554±130, 598±20, 631±44, and 673±40 °C. Two samples of the Shooting Creek schist were also analyzed; the first contains sillimanite and lacks muscovite, and yields a T estimate of 687±48 °C. The second sample contains rutile and sillimanite as inclusions in garnet, and yet yields widely scattered T values with a mean of 623±78 °C. All of the high-grade samples yield values that show poor internal consistency and are significantly lower than expected. These data suggest that Zr-in-rutile T estimates in high-grade rocks need careful consideration, and that considerable resetting of this system may be possible. These data also support the idea that the tectonothermal history of the highest grade rocks is significantly more complex than that for rocks at medium and lower grades. They were possibly affected by a second, later orogenic event such as the Acadian Orogeny (~350 Ma) now recognized to the east in the Piedmont, or perhaps these rocks experienced prolonged Taconic retrogression at high enough T to significantly affect the Zr-Ti system.